As is well known, a coiled magnet, if wound with wire possessing certain characteristics, can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold reduces the resistance in the magnet coils to negligible levels, such that when a power source is initially connected to the coil (for a period, for example, of 10 minutes) to introduce a current flow through the coils, the current will continue to flow through the coils due to the negligible resistance even after power is removed, thereby maintaining a magnetic field. Superconducting magnets find wide application, for example, in the field of magnetic resonance imaging (hereinafter "MRI").
A known superconducting magnet system comprises a circular cylindrical magnet cartridge having a plurality (e.g., three) of pairs of superconducting magnet coils; a toroidal inner cryostat vessel ("helium vessel") which surrounds the magnet cartridge and is filled with liquid helium for cooling the magnets; a toroidal low-temperature thermal radiation shield which surrounds the helium vessel; a toroidal high-temperature thermal radiation shield which surrounds the low-temperature thermal radiation shield; and a toroidal outer cryostat vessel ("vacuum vessel") which surrounds the high-temperature thermal radiation shield and is evacuated.
Since it is necessary to provide electrical energy to the main magnet coils, to various correction coils and to various gradient coils employed in MRI systems, there must be at least one penetration through the vessel walls. These penetrations must be designed to minimize thermal conduction between the vacuum vessel and the helium vessel, while maintaining the vacuum in the toroidal volume between the vacuum and helium vessels. In addition, the penetrations must compensate for differential thermal expansion and contraction of the vacuum and helium vessel. The penetration also serves as a flow path for helium gas in the event of a magnet quench, i.e., a magnet losing its superconductive state.
It is known to use a bellows as the magnet penetration tube. The convolutions of the bellows provide for additional thermal length (typically four times the straight length). However, even with the additional thermal length provided by the convolutions, the thermal conduction load from the bellows to the helium vessel can be significant (10-15% of the total heat load in some designs). Since it is the goal of the cryostat designer to minimize system boil-off, any reduction of the heat load can result in significant life-cycle cost reductions due to reduced helium consumption. Thus, there is a need to incorporate structural design features which reduce the heat load from the bellows to the helium vessel.